Complex carbohydrates play important roles in biological systems. However, it is very difficult to obtain these structures in pure homogeneous forms either by isolation from nature or by chemical synthesis. Therefore, the detailed structure-activity relationship is usually not clear, even for well known compounds with important functions. The ultimate goal of this project is to develop novel chemoenzymatic synthetic methods to efficiently obtain synthetically challenging carbohydrates and analogs in amounts large enough for structural characterization, functional studies, and therapeutic applications. For the current funding period, we are focusing on the synthesis of heparin and heparan sulfate (HS) oligosaccharide analogs. We hypothesize that N-sulfated analogs can mimic heparin/HS oligosaccharides for binding to their target proteins and have similar bioactivities. In addition, the N-sulfated analogs with synthetically controllable N- sulfation patterns can be powerful tools to probe the important roles of individual sulfations and provide critical information about individual roles of related O-sulfation in functional heparin/HS oligosaccharides. To test this hypothesis, we propose to synthesize a list of N3-modified GlcNAc or GlcA derivatives that can be used as substrates for UDP-GlcNAc and UDP-GlcA biosynthetic enzymes and heparosan synthases for producing N3- containing oligosaccharides. The azido group can then be reduced to an amino group and followed by chemical N-sulfation to provide N-sulfated analogs of heparin/HS oligosaccharides for testing their activities. Four specific aims are 1) chemical synthesis of N-acetylglucosamine (GlcNAc) and uronic acid derivatives as monosaccharide precursors; 2) synthesis of UDP-GlcNAc, UDP-uronic acids, and their derivatives; 3) enzymatic synthesis of heparin and heparan sulfate oligosaccharide analogs using heparosan synthases; 4) structure-activity relationship (SAR) studies using heparin/heparan sulfate-binding proteins. In addition, we will collaborate with our colleague and long-time collaborator, Prof. Andrew Fisher, an expert structural biologist with special expertise on protein crystal structural studies to solve the crystal structures of heparosan synthases, which are important enzymes for the assembly of heparin/HS polysaccharide structures. The information learned and the products obtained will facilitate the discovery and development of new therapeutics.